Reeling from three drinking water pollution incidents in the span of a month, environment officials said on Wednesday they are placing water quality at the top of their list of national environmental challenges this year.

"The outlook on water quality nationally is not optimistic, with 9 percent of the monitoring sections among the 10 major watersheds rated lower than Grade V, the worst level," Vice-Minister of Environmental Protection Li Ganjie said at a Wednesday news conference.

Experts have said the government must take tougher measures to protect sources of drinking water and expedite the construction of backup sources.

Though Li said the nation faces numerous problems with its drinking water sources, he also said more than 10 drinking water pollution incidents happen each year.

The recent string of major drinking water incidents began on April 10 in Lanzhou, Gansu province. City authorities detected excessive benzene in the tap water and shut down water lines for five days in some parts of the city, resulting in frenetic purchasing of liquids at supermarkets.

On April 23, authorities in Wuhan, Hubei province, suspended its tap water for more than 16 hours after excessive ammonia nitrogen was discovered in the Hanjiang River.

On May 9, the government of Jingjiang, a city along the Yangtze River in Jiangsu province, suspended its tap water sourced from the river for seven hours after a "pungent smell" was detected coming from the river.

"Having three incidents in a month has revealed that there are great risks with the urban tap water supply and problems with water sources," said Zhang Xiaojian, a professor on drinking water safety from the school of the environment at Tsinghua University.

He said that among all sources of drinking water - including rivers, reservoirs, and groundwater - rivers pose the greatest risk.

"Ideally, there should be no industrial facilities close to drinking water sources," he said. "But in China the biggest rivers have these facilities near them and they are potential risks to residents downstream."

China imposed new standards for drinking water on July 1, 2012, hiking the number of water quality indicators to 106 from 35.

But Zhang said fewer than 10 indicators among the 106 are officially tested on a daily basis. Some are tested once a month, twice a year, or every two years.

"The indicator that found high levels of benzene in Lanzhou is only tested twice a year because that substance does not normally exist in water," Zhang said.

Chen Ming, head of the water resources department at the Ministry of Water Resources, said to test all 106 indicators more frequently calls for better equipment and more personnel. Testing each indicator requires more financial investment that is beyond the capability of most official laboratories, Chen added.

"A more cost-effective method is to choose indicators that can detect chemicals or byproducts based on the industrial facilities in a region, and to test the selected indicators more frequently," Chen said.

Experts have also called for the construction of backup water sources.

According to the State Council in 2000, all cities with populations larger than 500,000 should find backup drinking water sources.

Lanzhou, a provincial capital with 3.62 million permanent residents, does not have a backup drinking water source, which is why residents had to wait for five days for the tap water lines to be reopened. For a city with a backup, the wait time is shortened significantly. Jingjiang resumed its water supply in only seven hours because it has a backup source.

Half of the cities in China have only one drinking water source. Once water pollution incidents occur, cities become paralyzed, said Li Yuanyuan, deputy head of the China Renewable Energy Engineering Institute, in a recent interview with Chinanews.com.

wuwencong@chinadaily.com.cn